The present invention relates to a method of maintaining the effect of the engine brake of a vehicle during hill descent by way of an active, electronically controlled intervention of the brakes on at least one wheel.
Road vehicles are known in the art that are equipped with devices which permit taking influence on the driving behavior of the vehicle, irrespective of the driver, by means of predefinable pressures in individual wheel brakes and by means of an intervention into the engine management of the drive engine. These devices relate to a brake slip control system (ABS) that is intended to prevent locking of individual wheels during a brake operation, a traction slip control system (TCS) that prevents spinning of the driven wheels, an electronic brake force distribution system (EBD) which controls the ratio of the brake forces between the front and rear axles of the vehicle, and an electronic driving stability control system (ESP) which ensures stable driving conditions in cornering maneuvers.
xe2x80x98Vehiclexe2x80x99 in this context refers to an automotive vehicle with four wheels which is equipped with a hydraulic brake system. The assembly units, which are already provided in the vehicle, such as the electronic system, wheel speed sensors, acceleration sensors, solenoid control valves, etc., are also used to implement the method of controlling the driving speed of a vehicle during hill descent, inasfar as these units are required for the said method. It goes without saying that these known assembly units can also be employed in a vehicle exclusively for the method of maintaining the effect of engine braking of a vehicle during hill descent.
WO 96/11826 A1 discloses a vehicle with a so-called hill descent control (also referred to as HDC=Hill Descent Control) which can be activated by a driver by means of a switch. This control is able to maintain the vehicle on a steep hill descent at a constant low speed by way of an active controlled braking operation without the driver needing to apply the brakes. This system is especially intended for use on off-road vehicles riding on a descending gradient which is so steep that the effect of engine braking is no longer sufficient to decelerate the vehicle, not even in the lowest gear. A speed controller compares the target speed with the actual speed and calculates therefrom a brake pressure for all four wheel brakes, by taking into consideration various marginal conditions.
Further, DE 198 17 212 A1 discloses a system for four-wheel drive vehicles with a central differential gear which provides a device for brake control by means of which either brake force is applied to a freely rotating wheel unable to transmit force onto a roadway, or to the two front wheels. The objective is to prevent an inadvertent acceleration due to the freely rotating wheel in a four-wheel permanently driven vehicle because no engine brake torque is transmitted under these conditions unless the central differential gear is locked by a central differential lock mechanism.
An object of the present invention is to provide a method of maintaining the effect of the engine brake of a vehicle during hill descent which safely prevents wheels that rotate in opposition to the driving direction and an acceleration of the vehicle.
This object is achieved by the features of claim 1.
Favorable improvements are indicated in the subclaims.
Due to the fact that in a method of maintaining the effect of engine braking of a vehicle during hill descent, a determination of the status of the wheels split into xe2x80x98stable wheelsxe2x80x99 and xe2x80x98unstable wheelsxe2x80x99 and the deceleration of all stable wheels in the event of at least one unstable wheel, on which latter the conditions
preferably Aunstable less than xe2x88x920.4 h
and Vunstable less than Kixc3x97Vref
occur, is effected by way of an active electronically controlled brake intervention on at least one wheel,
wherein
Vunstable=speed of the unstable wheel
Vref=filtered medium wheel speed of the stable wheels
Ki=correction factor (Ki less than 1)
Aunstable=acceleration of the unstable wheel.
The engine drive torque is transmitted onto the unstable wheel by way of the differential gear and, thus, reverse rotation of the unstable wheel is prevented because the rotation of the unstable wheel in a direction opposite to the stable wheels can only occur when no engine drive torque acts on the unstable wheels. The present invention is based on the reflection that the sum of the four wheel speeds remains equal due to the permanent engine rotational speed and the constant transmission ratio of the differential gears so that the wheels accelerate on a high coefficient of friction, while the wheels decelerate, stop and reverse on a low coefficient of friction. On xcexc-split descending gradients the low-xcexc wheels can reverse.
The status of the wheels is determined according to
Vrefxe2x88x92Kb less than Vstable less than Vref+Kb,
wherein
Vref=filtered medium wheel speed of the stable wheels
Kb=correction factor
Vstable=stable wheel.
The wheels are stable when the wheel speeds lie in a band around the medium wheel speed, that means, within a band which is determined by the correction factor Kb that lies in a band of 2.5 km/h, preferably 1.3 km/h, above and below the medium wheel speed. The wheels are unstable when the wheel speeds lie outside the above-mentioned band.
The medium wheel speed Vref is calculated from the filtered maximum value of the second fastest wheel and the vehicle speed, i.e., the low-pass filtered maximum of the second fastest wheel and the vehicle speed or the ABS reference speed is produced. Low-pass filtering of the medium wheel speed is effected at 2.5 hertz.
The correction factor Ki is smaller than xc2xd. Due to the fact that the status of an unstable wheel is lower than 50% of the medium wheel speed, the stable wheels are braked immediately when an unstable wheel is detected. An unstable wheel is unable to reverse under the condition which is decisive for the status determination, because an engine drive torque is applied to the unstable wheel before it can reverse in the event of a wheel speed of the unstable wheel that is lower than 50% of the medium wheel speed.
Due to the fact that the wheel acceleration of the unstable wheel now as before is lower than xe2x88x920.4 g, it is ensured that the danger of the unstable wheel reversing the direction of rotation, which may become real, is detected.
The pressure which must be introduced into the wheel brake cylinders of the wheel brakes for decelerating the stable wheels is determined according to
reqp=Kxc3x97(Vrefxe2x88x92Vref nom))+offset,
wherein
reqp=pressure requirement of the stable wheels
K=7 bar/(km/h)
Vref nom=average speed of all wheels before an unstable wheel is detected
offset=5 bar.
The pressure requirement of the stable wheels is calculated from the difference between the average speed of the stable wheels and the average speed of all wheels prior to the detection of an unstable wheel with a correction factor and offset factor.
Advantageously, the determinations of status are split up over a number of n time steps, and at least the values of the last status determination are stored. According to one embodiment, the pressure requirement of the preceding status determination is compared with the pressure requirement of the current status determination, and the current pressure requirement is overruled when the pressure requirement of the current status determination differs in its amount by less than pkp from the pressure requirement of the preceding status determination. The reference factor Kp less than 1 bar applies. Actuation of the hydraulic valves is prevented by this measure in order to enhance the control comfort and diminish noises.
In another embodiment, the unstable wheels of the preceding status determination are compared with the wheels of the current status determination, and upon deceleration of a preceding unstable wheel, the pressure build-up on this wheel is delayed. The delay of the pressure build-up on the preceding unstable wheel occurs within a period of time of 1 to 2 seconds, preferably, within 1.5 seconds. The control of the driving speed starts to operate only if the vehicle speed is  less than 30 km/h, preferably  less than 20 km/h, especially  less than 12 km/h, and no wheel brake is active.